1. Field of the Invention
This invention relates to an electrophotographic apparatus, a process cartridge and an electrophotographic photosensitive member unit.
2. Related Background Art
Various systems such as an electrophotographic system, a thermal-transfer system and an ink-jet system have conventionally been employed in image forming apparatus. Of these, an image forming apparatus employing the electrophotographic system, what is called an electrophotographic apparatus, has superiority to image forming apparatus employing other systems, in view of high speed, high image quality and noiselessness.
In addition, not only monochrome electrophotographic apparatus, but also polychrome (color) electrophotographic apparatus (color electrophotographic apparatus) have become popular.
Various systems are employed in such color electrophotographic apparatus. For example, well known are an intermediate transfer system in which exposure and development are successively performed for each color by means of a single electrophotographic photosensitive member, and respective color toner images are primarily sequentially transferred onto an intermediate transfer member (such as an intermediate transfer drum or an intermediate transfer belt), where the toner images thus transferred are thereafter secondarily transferred in a lump onto a transfer material to form a color image; an in-line system in which respective color toner images are respectively formed in respective color image forming sections disposed in series (each having an electrophotographic photosensitive member, a charging means, an exposure means, a developing means, a transfer means and so forth), and the toner images thus formed are sequentially transferred to a transfer material transported to the respective image forming sections in turn, to form a color image; and a multiple transfer system in which exposure and development are successively performed for each color by means of a single electrophotographic photosensitive member, and respective color toner images are sequentially transferred onto a transfer material (such as paper) held on a transfer material carrying member (such as a transfer drum), to form a color image.
Now, in recent years, various approaches have been taken because of an increasing need for the achievement of ultra-high resolution and ultra-high image quality with respect to the electrophotographic apparatus. Among various approaches, the relationship between an electrophotographic photosensitive member and an exposure means for forming an electrostatic latent image on the surface of the electrophotographic photosensitive member is considered to be particularly important because it is the basis of image formation. For example, Japanese Patent No. 3254833 (Patent Document 1) discloses, in a system making use of a laser beam as exposure light (imagewise exposure light), the relationship between the writing pitch of the laser beam and the total deflection of a cylindrical electrophotographic photosensitive member (photosensitive drum).
But, however fine the writing pitch of the laser beam, images with ultra-high resolution and ultra-high image quality are not obtainable unless a beam spot formed on the surface of the electrophotographic photosensitive member by a laser beam has a small spot diameter (beam-spot diameter).
A beam spot formed on the surface of the electrophotographic photosensitive member by a laser beam emitted from a laser of around 780 nm in oscillation wavelength (a near infrared semiconductor laser), having conventionally been used as an exposure light source of electrophotographic apparatus, has a spot diameter of about 100 μm, the limit of which is about 50 to 80 μm whatever improvements are made on various optical members.
Even if improvements on various optical members have made the beam spot have a small spot diameter, it is difficult to obtain the sharpness of the contour of the beam spot. This is known from the diffraction limit of laser beams that is represented by the following equation (1). The following equation (1) shows that the lower limit of the spot diameter (D) of a beam spot is proportional to the wavelength (λ) of the laser beam. (NA is the numerical aperture of a lens.)D=1.22λ/NA  (1)
Accordingly, in recent years, it is contemplated to use as an exposure light source a laser having a short oscillation wavelength (a blue semiconductor laser) (e.g., Japanese Patent Application Laid open No. H9 240051 (Patent Document 2)).
Where a laser having an oscillation wavelength within the range of from 380 nm to 450 nm is used as an exposure light source, the beam spot can be made to have a fairly small spot diameter (40 μm or less) in the state the sharpness of the contour of the beam spot is maintained. Hence, this enables achievement of ultra-high resolution, and is very advantageous for the achievement of ultra-high image quality.
Patent Document 1
Japanese Patent No. 3254833
Patent Document 2
Japanese Patent Application Laid open No. H9 240051